Mimicking pregnancy as a strategy for breast cancer prevention.

Pregnancy and its effects on breast cancer risk have been widely investigated; there is consensus among researchers that early pregnancy confers protection against breast cancer later in life, whereas nulliparity and late-age parity have been associated with increased risk of developing breast cancer. The answer to the question of how pregnancy reduces breast cancer risk has been elusive; however, pregnancy, like breast cancer, is a similar hormone-dependent entity under direct control of estrogen, progesterone and, of particular importance, human chorionic gonadotropin (hCG). In this report, we emphasize the main changes, previously described by our laboratory, in morphology and gene expression levels of the mammary gland of Sprague-Dawley rats exposed to known cancer-preventative conditions (pregnancy, hCG and progesterone + estrogen). In addition, we postulate a protective mechanism induced by hCG that could reduce the cell's potential to be transformed by carcinogens.

Human chorionic gonadotropin (hCG) prevents the transformed phenotypes induced by 17 beta-estradiol in human breast epithelial cells.

Human chorionic gonadotropin (hCG), a hormone produced during pregnancy, can elicit life-long refractoriness to carcinogenesis by differentiation of the breast epithelium. Human breast epithelial cells MCF-10F form tubules in collagen, mimicking the normal ductules. We have shown that 17 beta-estradiol (E2) alter the ductulogenic pattern of these cells. The effect of the recombinant hCG (rhCG) in vitro was evaluated on the transformation of MCF-10F induced by E2. MCF-10F cells were treated with 70 nM E2 alone or in combination with 50 IU/ml rhCG during 2 weeks, while the controls were treated with DMSO (the solvent in which E2 was dissolved) or rhCG alone. At the end of treatment, the cells were plated in type I collagen matrix (3D-cultures) for detecting 2 main phenotypes of cell transformation, namely the loss of ductulogenic capacity and the formation of solid masses. Although E2 significantly increased solid mass formation, this effect was prevented when MCF-10F cells were treated with E2 in combination with rhCG. Furthermore, E2 increased the main duct width (p < 0.001), and caused a disruption of the luminal architecture, whereas rhCG increased the length of the tubules (p < 0.001) and produced tertiary branching. In conclusion, rhCG was able to abrogate the transforming abilities of estradiol, and had the differentiating property by increasing the branching of the tubules formed by breast epithelial cells in collagen. These results further support our hypothesis, known as the terminal differentiation hypothesis of breast cancer prevention, that predicts that hCG treatment results in protection from tumorigenic changes by the loss of susceptible stem cells 1 through a differentiation to refractory stem cells 2 and increase differentiation of the mammary gland.

The concept of stem cell in the mammary gland and its implication in morphogenesis, cancer and prevention.

The breast attains its maximum development during pregnancy and lactation. After menopause the breast regresses in both nulliparous and parous women containing lobular structures that have been designated lobules type 1. Despite the similarity in the lobular composition of the breast at menopause, the fact that nulliparous women are at higher risk of developing breast cancer than parous women, indicates that Lobules type 1 in these two groups of women might be biologically different, or exhibit different susceptibility to carcinogenesis. Based on these observations it was postulated that the Lobule type 1 found in the breast of nulliparous women and of parous women with breast cancer never went through the process of differentiation, retaining a high concentration of epithelial cells that are targets for carcinogens and therefore susceptible to undergo neoplastic transformation, these cell are called Stem cells 1, whereas Lobules type 1 structures found in the breast of early parous postmenopausal women free of mammary pathology, on the other hand, are composed of an epithelial cell population that is refractory to transformation called Stem cells 2. It was further postulated that the degree of differentiation acquired through early pregnancy has changed the "genomic signature" that differentiates the Lobule type 1 from the early parous women from that of the nulliparous women by shifting the Stem cell 1 to a Stem cell 2 that is refractory to carcinogenesis, making this the postulated mechanism of protection conferred by early full term pregnancy. The identification of a putative breast stem cell (Stem cell 1) has reached in the last decade a significant impulse and several markers also reported for other tissues have been found in the mammary epithelial cells of both rodents and humans. Although still more work needs to be done in order to better understand the role of the Stem cell 2 and its interaction with the genes that confer it a specific signature, collectively, the data presently available provides evidence that pregnancy, through the process of cell differentiation, shifts the Stem cell 1 to Stem cell 2, cells that exhibit a specific genomic signature that could be responsible for the refractoriness of the mammary gland to carcinogenesis.